This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The ability to sustain aerobic exercise is an integrative response which relies on the interaction between oxygen delivery by the vasculature and the aerobic ATP production by the muscle mitochondria to maintain the high energetic demand of skeletal muscle during exercise. Skeletal muscle is dynamic and has the capacity to adapt to different perturbations such as exercise training, gene deletion, deconditioning, or aging. As a result, a series of metabolic and structural changes take place in muscle fibers and in the muscle vascular bed, which therefore can improve or diminish endurance performance during exercise. Two of the primary changes that allow for increased endurance during exercise are the mitochondrial biogenesis in skeletal muscle and the increased peripheral capillarization, which are associated with decreased muscle fatigability. Numerous non-exercise strategies for increasing muscular endurance have been proposed with very little success. Some of these include the use of natural products as sources of flavonoids. These compounds have gained attention because they show potential as endurance promoters (e.g. green tea extract or resveratrol). In the case of resveratrol, 15 weeks of treatment in mice show increased endurance treadmill performance and increased ADP-stimulated oxygen consumption which were associated with increased mitochondrial content. We will use advanced electron microscopy to examine the effects of cocoa flavonols on mitochondrial function and structure.